Many devices are presently used to measure light reflected from or transmitted through a surface in an attempt to determine certain qualities or characteristics of the subject surface and/or bulk. These types of optical measurement devices have a wide variety of applications, including: defect detection, quality control, and contamination assessment for surfaces and bulk materials such as mirrors, windows, semiconductors, crystals, coated surfaces, etc. In general, the prior art devices in this field can be classified into one of two categories: (1) expensive, accurate, research measurement devices; and (2) inexpensive, relatively inaccurate, field comparison devices.
The optical research measurement devices are large, relatively expensive apparatus having measurement volumes that exceed several cubic feet and costing between $50,000 to $500,000. These devices are usually permanently mounted on an optical table and employ computer controlled mechanical stages for measuring the reflected light from the sample. Because of the precise nature of the measurements, these type of apparatus require careful alignment and may take several hours to complete an entire set of measurements for a sample. In current state-of-the-art devices, the data measured using an optical research measurement device is generally presented in a nationally recognized format referred to as the Bidirectional Reflectance Distribution Function (BRDF). Some of these systems are also capable of further data analysis to provide sample characteristics, such as surface roughness.
Examples of these types of optical research measurement devices include a variety of custom built research measurement instruments for use in various industry and government optical laboratories as shown, for example, in U.S. Pat. Nos. 3,771,880, 3,971,956, 4,156,571, 4,360,275 and 4,859,062. Several research measurement scatterometers are also available from Breault Research Organization. The assignee of the present invention, TMA Technologies, Inc., also provides two types of optical research measurement instruments: the CASI.TM. line of instruments and the QwikScan.TM. line of instruments. In addition, several semiconductor manufacturers have created optical research measurement devices to determine the surface characteristics of semiconductor substrates, as shown, for example, in U.S. Pat. Nos. 4,097,160, 4,402,163, 4,583,861 and 4,632,561.
The inexpensive and less accurate field comparison devices generally perform some type of relative comparison whereby the value obtained by the comparison device is compared to a known good sample, rather than determining an absolute measurement of light intensity. Because the data from this type of device is taken in such a manner to make a relatively quick and inexpensive comparison determination, it is not possible to present the data in the BRDF format. Hence, no meaningful data analysis is possible other than the relative comparison determination.
Examples of these types of field comparison devices include surface quality systems developed to monitor material quality during manufacturing. Generally, such systems employ a scanning beam, and compare the relative signal changes with the signal from a known good sample to indicate the presence of a flaw. Examples of field comparison devices that use scattered light as a comparison to test windshields, for example, include a stray light measuring device as described in the SAE Technical Paper by A. Timmermann and G. Gehring, entitled "Field Measurement of Windshield Surface Wear" (September 1986), and U.S. Pat. Nos. 3,771,877 and 4,687,338. A variety of reflectometers have also been used as comparison devices for making simple optical comparisons, including U.S. Pat. Nos. 3,473,878, 4,373,819 and 4,552,458 and a paper by R. Hartman entitled "Field Test Method to Determine Window Replacement Criteria for Optical Systems" (May 1984).
Although present optical measurement devices have proven satisfactory for many applications, it would be desirable to provide an accurate, portable and relatively inexpensive measurement device for measuring light scatter that is capable of making accurate, absolute measurements, presenting data in BRDF format, and analyzing data to provide needed sample characteristics. In addition, it would be advantageous if such a device were capable of making measurements of light scatter for a wide variety of applications under a variety of environmental conditions.